Diversity combining is widely used within digital communications due to the performance gains which result from combining two or more separately faded receiver branches. In order to realize the entire available gain, the diversity branches must be accurately weighted and combined. To accomplish this, accurate information about a channel, or a signal transmitted, must be available to the diversity receiver. However, since the structure of the channel is typically unknown, the channel parameters required to realize the entire available gain must be estimated by the receiver.
For an M-branch diversity receiver for an arbitrary binary communication channel with time-varying channel gain and noise variance, the channel can be modeled as: EQU r.sub.m =p.sub.m x.sub.s +n.sub.m, m.epsilon.1. . . M
where r.sub.m is the received signal vector, p.sub.m is the channel gain (diagonal) matrix, x.sub.s is the transmitted signal vector, n.sub.m is the noise vector, and m denotes the diversity branch. The most general linear combiner can be modeled as ##EQU1## where .alpha..sub.m denotes the diversity weighting coefficient or parameter for branch m.
It can be shown that by defining an error signal e.sub.sm (k)=r.sub.m (k)-x.sub.s (k), the individual components of .alpha..sub.m (k) may be calculated as ##EQU2## As indicated by these equations, the validity of these estimates is directly related to the accuracy of .sigma..sub.rm.sup.2 (k) and .sigma..sub.em.sup.2 (k). While .sigma..sub.rm.sup.2 (k) is simply related to the received signal power, .sigma..sub.em.sup.2 (k) is not as easy to obtain since, at the receiver, the transmitted sequence x.sub.s (k) is not available. Current techniques attempt to circumvent this problem by assuming that for a specific symbol k in the received sequence of the signal, the error signal is the difference between the received signal and the closest constellation point (CCP). While this technique is adequate if the CCP corresponds to the transmitted signal, in cases where it does not (i.e., the channel has caused an error), the estimate of .sigma..sub.em.sup.2 (k) can be highly inaccurate and hence .alpha..sub.m (k) can be highly inaccurate.
Thus, a need exists for a new method and apparatus for estimating the diversity weighting coefficient .alpha..sub.m (k) which provides a significant increase in accuracy by fully utilizing the information available at the diversity receiver.